Measure a declaration against the bus's largest message, raise a condition near it, and send the vault's export compact (issue 1689)

A declaration is one bus message and the control node's passed the limit by 2 KB. The size is checked at every send
against the connection's own limit, with a condition from 90% and a message naming size and limit when over; the
export, 138 KB compact of 165 KB indented, is sent compact.

internal/link/bus.go

@@ -184,6 +184,14 @@ func (b OverNATS) PublishMembershipAfter(ctx context.Context, node, module strin
 	return nil
 }
 
+// MaxPayload is the largest message the connected server takes, as the server told this connection.
+func (b OverNATS) MaxPayload() int64 {
+	if b.Conn == nil {
+		return 0
+	}
+	return b.Conn.MaxPayload()
+}
+
 func (b OverNATS) PublishDeclaration(ctx context.Context, node string, body []byte) error {
 	_, err := b.JS.Publish(DeclareSubject(node), body, nats.Context(ctx))
 	if err != nil {